1. Field of the Invention
The present invention relates to a flat panel display controller, and more particularly, to a malfunction prevention technology for a source driver integrated circuit, which is capable of improving a malfunction occurring during the initial interval of a power sequence in a source driver integrated circuit mounted in a chip-on-glass (COG) form on a flat panel display.
2. Description of the Related Art
A flat panel displays receives data from a source device and displays the received data. The flat panel displays is used for various fields such as television and smart phone.
The flat panel displays include a liquid crystal display (LCD), an organic light emitting diodes (OLED) and so on. For example, the LCD generates an internal voltage required for operation and uses the generated voltage to drive a digital logic circuit or analog circuit.
The operation stability of internal circuits of the LCD may be guaranteed through a stable power sequence environment. For the operation stability, a variety of techniques have been adopted.
Korean Patent No. 10-0483534 may be taken as an example of a technique for seeking the stability of a power sequence environment of an LCD.
Korean Patent No. 10-0483534 has disclosed a technique for solving a problem which occurs when a low voltage used as digital power is applied after a high voltage used as analog power is applied to a source driver integrated circuit. According to this technique, when power is applied to a source driver integrated circuit of an LCD, a low voltage is first applied and a high voltage is then applied, in order to secure the reliability and stability of circuit elements.
As disclosed in Korean Patent No. 10-0483534, a general LCD employs the power sequence in which a low voltage is applied after a high voltage is applied.
In the flat panel displays such as the LCD, the source driver integrated circuit is mounted in a COG form in many cases. As such, the source driver integrated circuit mounted on glass is defined as a COG-form source driver integrated circuit. The COG-form source driver integrated circuit sequentially receives a high voltage and a low voltage from a printed circuit board (PCB) according to the power sequence.
When the COG-form source driver integrated circuit is mounted in the flat panel displays, the COG-form source driver integrated circuit may malfunction due to the power sequence of internal voltages provided from the PCB.
In general, the COG-form source driver integrated circuit includes buffers configured to output an even signal and an odd signal which have opposite polarities and a multiplexer configured to select one of the signals outputted from the buffers and output the selected signal. The multiplexer includes a plurality of transmission gates which are switched according to a control signal outputted from a level shifter. The multiplexer includes a transmission gate configured to isolate or equalize a line for outputting an even signal and a line for outputting an odd signal.
The level shifter includes a low-voltage logic and a high-voltage logic. The low-voltage logic is driven by a low voltage so as to process an input signal, and the high-voltage logic is driven by a high voltage so as to process an output signal.
In general, according to the power sequence, a low voltage is applied to the level shifter in a predetermined time after a high voltage is applied. Thus, the level shifter recognizes an input signal as a low level during an interval from the time at which the high voltage is applied to the time at which the low voltage is applied, and outputs a control signal as an output signal in a random state.
When the control signal outputted from the level shifter is maintained in a random state, all of the transmission gates of the multiplexer switched by the control signal of the level shifter may be turned on. As a result, the lines for outputting an odd signal and the lines for outputting an even signal in the buffers may be shorted by the turned-on transmission gates.
The high voltage and the low voltage which are provided to the COG-form source driver integrated circuit from the PCB include a driving voltage and a ground voltage, respectively. When the lines for outputting an odd signal and the lines for outputting an even signal in the buffers are shorted as described above, a terminal to provide a high-voltage driving voltage and a terminal to provide a high-voltage ground voltage in the PCB may be electrically connected to each other through the shorted path of the COG-form source driver integrated circuit.
Therefore, an over-current flow may occur through the shorted path of the COG-form source driver integrated circuit. The over current flows through the high-voltage driving voltage terminal, the shorted path of the COG-form source driver integrated circuit, and the high-voltage ground voltage terminal. At this time, a low-voltage driving voltage terminal and a low-voltage ground voltage terminal are isolated from the terminals for high voltages, and are not influenced by an over-current caused by the shorted path.
The COG-form source driver integrated circuit has a resistance component existing at a power source due to the characteristic of the COG form. Thus, in the COG-form source driver integrated circuit, the level of the high-voltage ground voltage rises due to an over-current caused by the shorted path within the multiplexer.
Although the input of the level shifter is normalized after a low voltage is applied according to the power sequence, a transistor included in the high voltage logic of the level shifter abnormally operates due to the rising level of the ground voltage. More specifically, a voltage difference between the gate and source of the transistor included in the high voltage logic does not exceed a threshold voltage due to the rising level of the ground voltage.
Therefore, the COG-form source driver integrated circuit of the flat panel display may malfunction due to the malfunction of the level shifter which is caused by the shorted path at the initial stage of the power sequence.
Furthermore, in the COG-form source driver integrated circuit of the flat panel displays, an over-current caused by the above-described shorted path may be continuously maintained. As a result, internal parts of the COG-form source driver integrated circuit may be damaged by the over-current.